Engine/generator/motor sets have heretofore been provided in a variety of applications and, in one field of application of such power sets, the internal combustion engine is a diesel engine, the generator is a direct current generator and the motor is a direct current motor.
Such drive systems can be used to propel industrial vehicles, especially floor vehicles such as forklift trucks.
With such vehicles, a drive pedal serves as a set point input for the internal combustion engine which, in the case of a Diesel engine, can have a fuel-injector pump serving as the speed-setting element. The speed-setting element is thus connected to the drive pedal or setpoint member.
The direct-current generator which is driven by the output shaft of the internal combustion engine can be of the externally energized type and the direct current motor can also be externally energized and connected to at least one wheel of the vehicle.
In such drive systems, it is customary in controlling the vehicle speed to either set the internal combustion engine to the desired speed, to selectively control the direct current generator, or to selectively control the current flow to the motor or the energizing field for the motor. This field may be supplied by a vehicle battery or from a supply network connected to electrical means. Customarily only one of these selectively controllable actions is required to adjust the vehicle speed.
Consequently, all of these systems apply techniques long known in the control of engine or electric motor systems, i.e. to regulate the speed of the engine or, where appropriate, the current flow or field energization of a drive motor.
Such systems, while effective for stationary applications and under other clearly determined operating conditions in which the actual speed is invariably a function of the pedal position, may not always be satisfactory.
There are occasions when problems have been encountered with such systems, some of which are more noticeable than others. For example, in a vehicle whose speed is exclusively controlled by the accelerator pedal, for example, the maintenance of the pedal in a fixed position may result in changes in the vehicle speed as a result of variations in the terrain or conditions encountered by the vehicle wheels. Thus, for instance, when the vehicle climbs hills or travels downwardly along an incline, the speed of the vehicle may change markedly in spite of the fact that the accelerator pedal is held in place.
For many vehicles it is crucial to be able to have extremely fine control of the actual speed of the vehicle and, indeed, to determine or establish this speed precisely, both when the position of the speed-control pedal is held constant a and when the travel resistance can change markedly.
This is especially the case for floor-type industrial vehicles, e.g. forklift trucks. Such vehicles are often called upon to transport especially sensitive equipment and articles as in the case of pallets loaded with glass bottles or flasks containing combustible or toxic liquids, and in such cases and for sensitive electronic equipment or the like it is absolutely essential that an extremely fine and sensitive control be provided during transportation, pickup and deposit of the load.
It has already been proposed to provide a vehicle drive including an internal combustion engine which acts upon a stepless, generally hydrostatic, transmission and in which the accelerator pedal produces a signal which affects the setting of the steplessly adjustable variable-ratio transmission.
In this system, a further signal can be generated to represent the speed of the internal combustion engine and still another signal can represent the load applied to the transmission, these signals being processed with the signal from the accelerator pedal (see German Patent Document DE-OS No. 20 49 048).
In this arrangement, however, the internal combustion engine speed signal and the transmission load signal do not provide continuous effects upon the setting of the steplessly adjustable transmission but rather form limiting signals for controlling the acceleration and, therefore, preventing excessive acceleration which would carry the signals beyond certain limits.
U.S. Pat. No. 4,135,121, moreover, discloses a vehicle drive system in which the vehicle is propelled by a battery-fed externally excited electric motor through a steplessly variable hydrostatic transmission. In this case, the excitation of the shunt wound motor is made dependent upon the load upon the stepless transmission. In this case as well, the setting of the transmission on the one hand and the setting of the prime mover or propelling motor on the other can be independent and only under certain conditions will be superimposed one on the other or correlated.